This invention relates to tire protective chains for heavy-duty earth-moving vehicles fitted with rubber tires, the chains being used to protect the tire treads from damage, such as abrasion and cutting of the rubber, by rocks.
In a popular firm the tire protective chain comprises a network of tread links forming a pattern of closed figures and connectors engaged with the interconnecting the tread links at the corners of the figures. The links are preferably held to be substantially perpendicular to and the connectors substantially parallel to the surface of the tire. Each link may be a loop or, preferably, is a plate formed with a pair of perforations or a closed slot. Each connector is formed from a rod that is bent into a loop. The connectors are threaded through the perforations in the links, usually three or four links to each connector, before being closed by a bridging member or by welding.
Tire protective chains have a number of requirements. They must be strong, reliable, and hard-wearing and they must be suitable for production on an economic scale. As will be explained below, it has not, up till now, been possible to satisfy these requirements simultaneously in an acceptable manner. Satisfying these requirements involves, firstly, using welded rather than non-welded connectors. This is because welded connectors have been found to give a greater strength than and to be less expensive than non-welded connectors. Furthermore non-welded connectors limit undesirably the attainable density of the net. Secondly, satisfying these requirements involves choosing materials for the connectors and the links which will have suitable wearing characteristics and amenable to fabrication.
As far as the tread links are concerned there is little difficulty as they do not have to be welded and the material such as a heat-treated through-hardened carbon steel, cab be chosen for its wearing properties, e.g. see U.S. Pat. No. 3,870,094 (Schurle et al).
For the connectors the problem is not at all simple. Up till now butt-welded mild steel connectors that have been carburized or case-hardened have been used e.g. see U.S. Pat. No. 3,943,990 (Rieger). This is not because they are preferred but because of the serious practical difficulties of using carbon steel as discussed by Rieger.
These are a number of reasons whay carbon steel connectors have been found to be impracticable to weld. On the one hand it has been found to be unsuccessful to butt-weld carbon steel connectors by deposition welding because of the difficulty of performing the weld on a connector which may be up to 18 mm in cross-section, because such a weld is unreliable, and because this type of welding is labour intensive and commercially unattractive. On the other hand the welding can be made commercially attractive by forming the butt-weld using resistance welding requiment. However the machinery for this involves a considerable capital outlay, is not easily transportable, and cannot be operated away from a large power supply. A further and more significant problem of this technique is that during welding material to either side of the weld is converted to mild steel so that after treating, while the body of the connector is through-hardened, the weld and material to either side of the weld is merely case-hardened and there is no significant improvement over the use of mild steel.
For the above-discussed reasons case-hardened, mild steel, welded connectors have been used conventionally, but the applicant has found that these have a major disadvantage in that once the carburized zone has worn away and the mild steel exposed, the connectors wear and deteriorate rapidly so resulting in failure of the chain.
It is an object of the invention to provide a tire protective chain which satisfactorily resolves the problems of conventional protective chains.